Image data encoding and decoding

ABSTRACT

An image data encoding apparatus includes an entropy encoder configured to selectively encode data items representing image data by an encoding system selected from a context adaptive binary arithmetic coding (CABAC) encoding system and a bypass encoding system. The data items include (i) a first data item indicating whether a multi-direction linear model (MDLM) chroma mode applies to a current image region; and (ii) a second data item indicating a processing direction for operation of the MDLM chroma mode. The entropy encoder encodes instances of the first data item and instances of the second data item independently of the encoding of instances of the first data item.

BACKGROUND Field

This disclosure relates to image data encoding and decoding.

Description of Related Art

The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, is neither expressly or impliedly admitted as prior art against the present disclosure.

There are several video data encoding and decoding systems which involve transforming video data into a frequency domain representation, quantising the frequency domain coefficients and then applying some form of entropy encoding to the quantised coefficients. This can achieve compression of the video data. A corresponding decoding or decompression technique is applied to recover a reconstructed version of the original video data.

High Efficiency Video Coding (HEVC), also known as H.265 or MPEG-H Part 2, is a proposed successor to H.264/MPEG-4 AVC. It is intended for HEVC to improve video quality and double the data compression ratio compared to H.264, and for it to be scalable from 128×96 to 7680×4320 pixels resolution, roughly equivalent to bit rates ranging from 128 kbit/s to 800 M bit/s.

SUMMARY

The present disclosure addresses or mitigates problems arising from this processing.

The present disclosure provides image data encoding apparatus, comprising:

an entropy encoder configured to selectively encode data items representing image data by an encoding system selected from a context adaptive binary arithmetic coding (CABAC), encoding system and a bypass encoding system;

in which the data items include (i) a first data item indicating whether a multi-direction linear model (MDLM) chroma mode applies to a current image region; and (ii) a second data item indicating a processing direction for operation of the MDLM chroma mode;

in which the entropy encoder is configured to encode instances of the first data item and to encode instances of the second data item independently of the encoding of instances of the first data item.

The present disclosure also provides an image data encoding method, comprising: selectively encoding data items representing image data by an encoding system selected from a context adaptive binary arithmetic coding (CABAC), encoding system and a bypass encoding system;

in which the data items include (i) a first data item indicating whether a multi-direction linear model (MDLM) chroma mode applies to a current image region; and (ii) a second data item indicating a processing direction for operation of the MDLM chroma mode;

in which the encoding step comprises encoding instances of the first data item and encoding instances of the second data item independently of the encoding of instances of the first data item.

The present disclosure also provides image data decoding apparatus, comprising:

an entropy decoder configured to selectively decode data items representing image data by a decoding system selected from a context adaptive binary arithmetic coding (CABAC), decoding system and a bypass decoding system;

in which the data items include (i) a first data item indicating whether a multi-direction linear model (MDLM) chroma mode applies to a current image region; and (ii) a second data item indicating a processing direction for operation of the MDLM chroma mode;

in which the entropy decoder is configured to decode instances of the first data item and to decode instances of the second data item independently of the decoding of instances of the first data item.

The present disclosure also provides an image data decoding method, comprising:

selectively decoding data items representing image data by a decoding system selected from a context adaptive binary arithmetic coding (CABAC), decoding system and a bypass decoding system;

in which the data items include (i) a first data item indicating whether a multi-direction linear model (MDLM) chroma mode applies to a current image region; and (ii) a second data item indicating a processing direction for operation of the MDLM chroma mode;

in which the decoding step comprises decoding instances of the first data and to decode instances of the second data item independently of the decoding of instances of the first data item.

Further respective aspects and features of the present disclosure are defined in the appended claims.

It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the present technology.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 schematically illustrates an audio/video (AN) data transmission and reception system using video data compression and decompression;

FIG. 2 schematically illustrates a video display system using video data decompression;

FIG. 3 schematically illustrates an audio/video storage system using video data compression and decompression;

FIG. 4 schematically illustrates a video camera using video data compression;

FIGS. 5 and 6 schematically illustrate storage media;

FIG. 7 provides a schematic overview of a video data compression and decompression apparatus;

FIG. 8 schematically illustrates a predictor;

FIG. 9 schematically illustrates a partially-encoded image;

FIG. 10 schematically illustrates a set of possible intra-prediction directions;

FIG. 11 schematically illustrates a set of prediction modes;

FIG. 12 schematically illustrates another set of prediction modes;

FIG. 13 schematically illustrates an intra-prediction process;

FIG. 14 schematically illustrates a CABAC encoder;

FIGS. 15 and 16 schematically illustrate CABAC encoding techniques;

FIGS. 17 and 18 schematically illustrate CABAC decoding techniques; and

FIGS. 19 and 20 are schematic flowcharts illustrating respective methods.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, FIGS. 1-4 are provided to give schematic illustrations of apparatus or systems making use of the compression and/or decompression apparatus to be described below in connection with embodiments of the present technology.

All of the data compression and/or decompression apparatus to be described below may be implemented in hardware, in software running on a general-purpose data processing apparatus such as a general-purpose computer, as programmable hardware such as an application specific integrated circuit (ASIC) or field programmable gate array (FPGA) or as combinations of these. In cases where the embodiments are implemented by software and/or firmware, it will be appreciated that such software and/or firmware, and non-transitory data storage media by which such software and/or firmware are stored or otherwise provided, are considered as embodiments of the present technology.

FIG. 1 schematically illustrates an audio/video data transmission and reception system using video data compression and decompression.

An input audio/video signal 10 is supplied to a video data compression apparatus 20 which compresses at least the video component of the audio/video signal 10 for transmission along a transmission route 30 such as a cable, an optical fibre, a wireless link or the like. The compressed signal is processed by a decompression apparatus 40 to provide an output audio/video signal 50. For the return path, a compression apparatus 60 compresses an audio/video signal for transmission along the transmission route 30 to a decompression apparatus 70.

The compression apparatus 20 and decompression apparatus 70 can therefore form one node of a transmission link. The decompression apparatus 40 and decompression apparatus 60 can form another node of the transmission link. Of course, in instances where the transmission link is uni-directional, only one of the nodes would require a compression apparatus and the other node would only require a decompression apparatus.

FIG. 2 schematically illustrates a video display system using video data decompression. In particular, a compressed audio/video signal 100 is processed by a decompression apparatus 110 to provide a decompressed signal which can be displayed on a display 120. The decompression apparatus 110 could be implemented as an integral part of the display 120, for example being provided within the same casing as the display device. Alternatively, the decompression apparatus 110 maybe provided as (for example) a so-called set top box (STB), noting that the expression “set-top” does not imply a requirement for the box to be sited in any particular orientation or position with respect to the display 120; it is simply a term used in the art to indicate a device which is connectable to a display as a peripheral device.

FIG. 3 schematically illustrates an audio/video storage system using video data compression and decompression. An input audio/video signal 130 is supplied to a compression apparatus 140 which generates a compressed signal for storing by a store device 150 such as a magnetic disk device, an optical disk device, a magnetic tape device, a solid state storage device such as a semiconductor memory or other storage device. For replay, compressed data is read from the storage device 150 and passed to a decompression apparatus 160 for decompression to provide an output audio/video signal 170.

It will be appreciated that the compressed or encoded signal, and a storage medium such as a machine-readable non-transitory storage medium, storing that signal, are considered as embodiments of the present technology.

FIG. 4 schematically illustrates a video camera using video data compression. In FIG. 4, an image capture device 180, such as a charge coupled device (CCD) image sensor and associated control and read-out electronics, generates a video signal which is passed to a compression apparatus 190. A microphone (or plural microphones) 200 generates an audio signal to be passed to the compression apparatus 190. The compression apparatus 190 generates a compressed audio/video signal 210 to be stored and/or transmitted (shown generically as a schematic stage 220).

The techniques to be described below relate primarily to video data compression and decompression. It will be appreciated that many existing techniques may be used for audio data compression in conjunction with the video data compression techniques which will be described, to generate a compressed audio/video signal. Accordingly, a separate discussion of audio data compression will not be provided. It will also be appreciated that the data rate associated with video data, in particular broadcast quality video data, is generally very much higher than the data rate associated with audio data (whether compressed or uncompressed). It will therefore be appreciated that uncompressed audio data could accompany compressed video data to form a compressed audio/video signal. It will further be appreciated that although the present examples (shown in FIGS. 1-4) relate to audio/video data, the techniques to be described below can find use in a system which simply deals with (that is to say, compresses, decompresses, stores, displays and/or transmits) video data. That is to say, the embodiments can apply to video data compression without necessarily having any associated audio data handling at all.

FIG. 4 therefore provides an example of a video capture apparatus comprising an image sensor and an encoding apparatus of the type to be discussed below. FIG. 2 therefore provides an example of a decoding apparatus of the type to be discussed below and a display to which the decoded images are output.

A combination of FIGS. 2 and 4 may provide a video capture apparatus comprising an image sensor 180 and encoding apparatus 190, decoding apparatus 110 and a display 120 to which the decoded images are output.

FIGS. 5 and 6 schematically illustrate storage media, which store (for example) the compressed data generated by the apparatus 20, 60, the compressed data input to the apparatus 110 or the storage media or stages 150, 220. FIG. 5 schematically illustrates a disc storage medium such as a magnetic or optical disc, and FIG. 6 schematically illustrates a solid state storage medium such as a flash memory. Note that FIGS. 5 and 6 can also provide examples of non-transitory machine-readable storage media which store computer software which, when executed by a computer, causes the computer to carry out one or more of the methods to be discussed below.

Therefore, the above arrangements provide examples of video storage, capture, transmission or reception apparatuses embodying any of the present techniques.

FIG. 7 provides a schematic overview of a video data compression and decompression apparatus.

A controller 343 controls the overall operation of the apparatus and, in particular when referring to a compression mode, controls a trial encoding processes by acting as a selector to select various modes of operation such as block sizes and shapes, and whether the video data is to be encoded losslessly or otherwise. The controller is considered to part of the image encoder or image decoder (as the case may be). Successive images of an input video signal 300 are supplied to an adder 310 and to an image predictor 320. The image predictor 320 will be described below in more detail with reference to FIG. 8. The image encoder or decoder (as the case may be) plus the intra-image predictor of FIG. 8 may use features from the apparatus of FIG. 7. This does not mean that the image encoder or decoder necessarily requires every feature of FIG. 7 however.

The adder 310 in fact performs a subtraction (negative addition) operation, in that it receives the input video signal 300 on a “+” input and the output of the image predictor 320 on a “-” input, so that the predicted image is subtracted from the input image. The result is to generate a so-called residual image signal 330 representing the difference between the actual and projected images.

One reason why a residual image signal is generated is as follows. The data coding techniques to be described, that is to say the techniques which will be applied to the residual image signal, tend to work more efficiently when there is less “energy” in the image to be encoded. Here, the term “efficiently” refers to the generation of a small amount of encoded data; for a particular image quality level, it is desirable (and considered “efficient”) to generate as little data as is practicably possible. The reference to “energy” in the residual image relates to the amount of information contained in the residual image. If the predicted image were to be identical to the real image, the difference between the two (that is to say, the residual image) would contain zero information (zero energy) and would be very easy to encode into a small amount of encoded data. In general, if the prediction process can be made to work reasonably well such that the predicted image content is similar to the image content to be encoded, the expectation is that the residual image data will contain less information (less energy) than the input image and so will be easier to encode into a small amount of encoded data.

The remainder of the apparatus acting as an encoder (to encode the residual or difference image) will now be described. The residual image data 330 is supplied to a transform unit or circuitry 340 which generates a discrete cosine transform (DCT) representation of blocks or regions of the residual image data. The DCT technique itself is well known and will not be described in detail here. Note also that the use of DCT is only illustrative of one example arrangement. Other transforms which might be used include, for example, the discrete sine transform (DST). A transform could also comprise a sequence or cascade of individual transforms, such as an arrangement in which one transform is followed (whether directly or not) by another transform. The choice of transform may be determined explicitly and/or be dependent upon side information used to configure the encoder and decoder.

The output of the transform unit 340, which is to say, a set of DCT coefficients for each transformed block of image data, is supplied to a quantiser 350. Various quantisation techniques are known in the field of video data compression, ranging from a simple multiplication by a quantisation scaling factor through to the application of complicated lookup tables under the control of a quantisation parameter. The general aim is twofold. Firstly, the quantisation process reduces the number of possible values of the transformed data. Secondly, the quantisation process can increase the likelihood that values of the transformed data are zero. Both of these can make the entropy encoding process, to be described below, work more efficiently in generating small amounts of compressed video data.

A data scanning process is applied by a scan unit 360. The purpose of the scanning process is to reorder the quantised transformed data so as to gather as many as possible of the non-zero quantised transformed coefficients together, and of course therefore to gather as many as possible of the zero-valued coefficients together. These features can allow so-called run-length coding or similar techniques to be applied efficiently. So, the scanning process involves selecting coefficients from the quantised transformed data, and in particular from a block of coefficients corresponding to a block of image data which has been transformed and quantised, according to a “scanning order” so that (a) all of the coefficients are selected once as part of the scan, and (b) the scan tends to provide the desired reordering. One example scanning order which can tend to give useful results is a so-called up-right diagonal scanning order.

The scanned coefficients are then passed to an entropy encoder (EE) 370. Again, various types of entropy encoding may be used. Two examples are variants of the so-called CABAC (Context Adaptive Binary Arithmetic Coding) system and variants of the so-called CAVLC (Context Adaptive Variable-Length Coding) system. In general terms, CABAC is considered to provide a better efficiency, and in some studies has been shown to provide a 10-20% reduction in the quantity of encoded output data for a comparable image quality compared to CAVLC. However, CAVLC is considered to represent a much lower level of complexity (in terms of its implementation) than CABAC. Note that the scanning process and the entropy encoding process are shown as separate processes, but in fact can be combined or treated together. That is to say, the reading of data into the entropy encoder can take place in the scan order. Corresponding considerations apply to the respective inverse processes to be described below.

The output of the entropy encoder 370, along with additional data (mentioned above and/or discussed below), for example defining the manner in which the predictor 320 generated the predicted image, provides a compressed output video signal 380.

However, a return path is also provided because the operation of the predictor 320 itself depends upon a decompressed version of the compressed output data.

The reason for this feature is as follows. At the appropriate stage in the decompression process (to be described below) a decompressed version of the residual data is generated. This decompressed residual data has to be added to a predicted image to generate an output image (because the original residual data was the difference between the input image and a predicted image). In order that this process is comparable, as between the compression side and the decompression side, the predicted images generated by the predictor 320 should be the same during the compression process and during the decompression process. Of course, at decompression, the apparatus does not have access to the original input images, but only to the decompressed images. Therefore, at compression, the predictor 320 bases its prediction (at least, for inter-image encoding) on decompressed versions of the compressed images.

The entropy encoding process carried out by the entropy encoder 370 is considered (in at least some examples) to be “lossless”, which is to say that it can be reversed to arrive at exactly the same data which was first supplied to the entropy encoder 370. So, in such examples the return path can be implemented before the entropy encoding stage. Indeed, the scanning process carried out by the scan unit 360 is also considered lossless, but in the present embodiment the return path 390 is from the output of the quantiser 350 to the input of a complimentary inverse quantiser 420. In instances where loss or potential loss is introduced by a stage, that stage may be included in the feedback loop formed by the return path. For example, the entropy encoding stage can at least in principle be made lossy, for example by techniques in which bits are encoded within parity information. In such an instance, the entropy encoding and decoding should form part of the feedback loop.

In general terms, an entropy decoder 410, the reverse scan unit 400, an inverse quantiser 420 and an inverse transform unit or circuitry 430 provide the respective inverse functions of the entropy encoder 370, the scan unit 360, the quantiser 350 and the transform unit 340. For now, the discussion will continue through the compression process; the process to decompress an input compressed video signal will be discussed separately below.

In the compression process, the scanned coefficients are passed by the return path 390 from the quantiser 350 to the inverse quantiser 420 which carries out the inverse operation of the scan unit 360. An inverse quantisation and inverse transformation process are carried out by the units 420, 430 to generate a compressed-decompressed residual image signal 440.

The image signal 440 is added, at an adder 450, to the output of the predictor 320 to generate a reconstructed output image 460. This forms one input to the image predictor 320, as will be described below.

Turning now to the process applied to decompress a received compressed video signal 470, the signal is supplied to the entropy decoder 410 and from there to the chain of the reverse scan unit 400, the inverse quantiser 420 and the inverse transform unit 430 before being added to the output of the image predictor 320 by the adder 450. So, at the decoder side, the decoder reconstructs a version of the residual image and then applies this (by the adder 450) to the predicted version of the image (on a block by block basis) so as to decode each block. In straightforward terms, the output 460 of the adder 450 forms the output decompressed video signal 480. In practice, further filtering may optionally be applied (for example, by a filter 560 shown in FIG. 8 but omitted from FIG. 7 for clarity of the higher level diagram of FIG. 7) before the signal is output.

The apparatus of FIGS. 7 and 8 can act as a compression (encoding) apparatus or a decompression (decoding) apparatus. The functions of the two types of apparatus substantially overlap. The scan unit 360 and entropy encoder 370 are not used in a decompression mode, and the operation of the predictor 320 (which will be described in detail below) and other units follow mode and parameter information contained in the received compressed bit-stream rather than generating such information themselves.

FIG. 8 schematically illustrates the generation of predicted images, and in particular the operation of the image predictor 320.

There are two basic modes of prediction carried out by the image predictor 320: so-called intra-image prediction and so-called inter-image, or motion-compensated (MC), prediction. At the encoder side, each involves detecting a prediction direction in respect of a current block to be predicted, and generating a predicted block of samples according to other samples (in the same (intra) or another (inter) image). By virtue of the units 310 or 450, the difference between the predicted block and the actual block is encoded or applied so as to encode or decode the block respectively.

(At the decoder, or at the reverse decoding side of the encoder, the detection of a prediction direction may be in response to data associated with the encoded data by the encoder, indicating which direction was used at the encoder. Or the detection may be in response to the same factors as those on which the decision was made at the encoder).

Intra-image prediction bases a prediction of the content of a block or region of the image on data from within the same image. This corresponds to so-called I-frame encoding in other video compression techniques. In contrast to I-frame encoding, however, which involves encoding the whole image by intra-encoding, in the present embodiments the choice between intra- and inter-encoding can be made on a block-by-block basis, though in other embodiments the choice is still made on an image-by-image basis.

Motion-compensated prediction is an example of inter-image prediction and makes use of motion information which attempts to define the source, in another adjacent or nearby image, of image detail to be encoded in the current image. Accordingly, in an ideal example, the contents of a block of image data in the predicted image can be encoded very simply as a reference (a motion vector) pointing to a corresponding block at the same or a slightly different position in an adjacent image.

A technique known as “block copy” prediction is in some respects a hybrid of the two, as it uses a vector to indicate a block of samples at a position displaced from the currently predicted block within the same image, which should be copied to form the currently predicted block.

Returning to FIG. 8, two image prediction arrangements (corresponding to intra- and inter-image prediction) are shown, the results of which are selected by a multiplexer 500 under the control of a mode signal 510 (for example, from the controller 343) so as to provide blocks of the predicted image for supply to the adders 310 and 450. The choice is made in dependence upon which selection gives the lowest “energy” (which, as discussed above, may be considered as information content requiring encoding), and the choice is signalled to the decoder within the encoded output data-stream. Image energy, in this context, can be detected, for example, by carrying out a trial subtraction of an area of the two versions of the predicted image from the input image, squaring each pixel value of the difference image, summing the squared values, and identifying which of the two versions gives rise to the lower mean squared value of the difference image relating to that image area. In other examples, a trial encoding can be carried out for each selection or potential selection, with a choice then being made according to the cost of each potential selection in terms of one or both of the number of bits required for encoding and distortion to the picture.

The actual prediction, in the intra-encoding system, is made on the basis of image blocks received as part of the signal 460, which is to say, the prediction is based upon encoded-decoded image blocks in order that exactly the same prediction can be made at a decompression apparatus. However, data can be derived from the input video signal 300 by an intra-mode selector 520 to control the operation of the intra-image predictor 530.

For inter-image prediction, a motion compensated (MC) predictor 540 uses motion information such as motion vectors derived by a motion estimator 550 from the input video signal 300. Those motion vectors are applied to a processed version of the reconstructed image 460 by the motion compensated predictor 540 to generate blocks of the inter-image prediction.

Accordingly, the units 530 and 540 (operating with the estimator 550) each act as detectors to detect a prediction direction in respect of a current block to be predicted, and as a generator to generate a predicted block of samples (forming part of the prediction passed to the units 310 and 450) according to other samples defined by the prediction direction.

The processing applied to the signal 460 will now be described. Firstly, the signal is optionally filtered by a filter unit 560, which will be described in greater detail below. This involves applying a “deblocking” filter to remove or at least tend to reduce the effects of the block-based processing carried out by the transform unit 340 and subsequent operations. A sample adaptive offsetting (SAO) filter may also be used. Also, an adaptive loop filter is optionally applied using coefficients derived by processing the reconstructed signal 460 and the input video signal 300. The adaptive loop filter is a type of filter which, using known techniques, applies adaptive filter coefficients to the data to be filtered. That is to say, the filter coefficients can vary in dependence upon various factors. Data defining which filter coefficients to use is included as part of the encoded output data-stream.

The filtered output from the filter unit 560 in fact forms the output video signal 480 when the apparatus is operating as a decompression apparatus. It is also buffered in one or more image or frame stores 570; the storage of successive images is a requirement of motion compensated prediction processing, and in particular the generation of motion vectors. To save on storage requirements, the stored images in the image stores 570 may be held in a compressed form and then decompressed for use in generating motion vectors. For this particular purpose, any known compression/decompression system may be used. The stored images are passed to an interpolation filter 580 which generates a higher resolution version of the stored images; in this example, intermediate samples (sub-samples) are generated such that the resolution of the interpolated image is output by the interpolation filter 580 is 4 times (in each dimension) that of the images stored in the image stores 570 for the luminance channel of 4:2:0 and 8 times (in each dimension) that of the images stored in the image stores 570 for the chrominance channels of 4:2:0. The interpolated images are passed as an input to the motion estimator 550 and also to the motion compensated predictor 540.

The way in which an image is partitioned for compression processing will now be described. At a basic level, an image to be compressed is considered as an array of blocks or regions of samples. The splitting of an image into such blocks or regions can be carried out by a decision tree, such as that described in Bross et al: “High Efficiency Video Coding (HEVC) text specification draft 6”, JCTVC-H1003_d0 (November 2011), and “Versatile Video Coding (Draft 5)”, JVET-N1001v8, the contents of which are incorporated herein by reference. In some examples, the resulting blocks or regions have sizes and, in some cases, shapes which, by virtue of the decision tree, can generally follow the disposition of image features within the image. This in itself can allow for an improved encoding efficiency because samples representing or following similar image features would tend to be grouped together by such an arrangement. In some examples, square blocks or regions of different sizes (such as 4×4 samples up to, say, 64×64 or larger blocks) are available for selection. In other example arrangements, blocks or regions of different shapes such as rectangular blocks (for example, vertically or horizontally oriented) can be used. Other non-square and non-rectangular blocks are envisaged. The result of the division of the image into such blocks or regions is (in at least the present examples) that each sample of an image is allocated to one, and only one, such block or region.

The intra-prediction process will now be discussed. In general terms, intra-prediction involves generating a prediction of a current block of samples from previously-encoded and decoded samples in the same image.

FIG. 9 schematically illustrates a partially encoded image 800. Here, the image is being encoded from top-left to bottom-right on a block by block basis. An example block encoded partway through the handling of the whole image is shown as a block 810. A shaded region 820 above and to the left of the block 810 has already been encoded. The intra-image prediction of the contents of the block 810 can make use of any of the shaded area 820 but cannot make use of the unshaded area below that.

In some examples, the image is encoded on a block by block basis such that larger blocks (referred to as coding units or CUs) are encoded in an order such as the order discussed with reference to FIG. 9. Within each CU, there is the potential (depending on the block splitting process that has taken place) for the CU to be handled as a set of two or more smaller blocks or transform units (TUs). This can give a hierarchical order of encoding so that the image is encoded on a CU by CU basis, and each CU is potentially encoded on a TU by TU basis. Note however that for an individual TU within the current coding tree unit (the largest node in the tree structure of block division), the hierarchical order of encoding (CU by CU then TU by TU) discussed above means that there may be previously encoded samples in the current CU and available to the coding of that TU which are, for example, above-right or below-left of that TU.

The block 810 represents a CU; as discussed above, for the purposes of intra-image prediction processing, this may be subdivided into a set of smaller units. An example of a current TU 830 is shown within the CU 810. More generally, the picture is split into regions or groups of samples to allow efficient coding of signalling information and transformed data. The signalling of the information may require a different tree structure of sub-divisions to that of the transform, and indeed that of the prediction information or the prediction itself. For this reason, the coding units may have a different tree structure to that of the transform blocks or regions, the prediction blocks or regions and the prediction information. In some examples such as HEVC the structure can be a so-called quad tree of coding units, whose leaf nodes contain one or more prediction units and one or more transform units; the transform units can contain multiple transform blocks corresponding to luma and chroma representations of the picture, and prediction could be considered to be applicable at the transform block level. In examples, the parameters applied to a particular group of samples can be considered to be predominantly defined at a block level, which is potentially not of the same granularity as the transform structure.

The intra-image prediction takes into account samples coded prior to the current TU being considered, such as those above and/or to the left of the current TU. Source samples, from which the required samples are predicted, may be located at different positions or directions relative to the current TU. To decide which direction is appropriate for a current prediction unit, the mode selector 520 of an example encoder may test all combinations of available TU structures for each candidate direction and select the prediction direction and TU structure with the best compression efficiency.

The picture may also be encoded on a “slice” basis. In one example, a slice is a horizontally adjacent group of CUs. But in more general terms, the entire residual image could form a slice, or a slice could be a single CU, or a slice could be a row of CUs, and so on. Slices can give some resilience to errors as they are encoded as independent units. The encoder and decoder states are completely reset at a slice boundary. For example, intra-prediction is not carried out across slice boundaries; slice boundaries are treated as image boundaries for this purpose.

FIG. 10 schematically illustrates a set of possible (candidate) prediction directions. The full set of candidate directions is available to a prediction unit. The directions are determined by horizontal and vertical displacement relative to a current block position, but are encoded as prediction “modes”, a set of which is shown in FIG. 11. Note that the so-called DC mode represents a simple arithmetic mean of the surrounding upper and left-hand samples. Note also that the set of directions shown in FIG. 10 is just one example; in other examples, a set of (for example) 65 angular modes plus DC and planar (a full set of 67 modes) as shown schematically in FIG. 12 makes up the full set. Other numbers of modes could be used.

In general terms, after detecting a prediction direction, the systems are operable to generate a predicted block of samples according to other samples defined by the prediction direction. In examples, the image encoder is configured to encode data identifying the prediction direction selected for each sample or region of the image (and the image decoder is configured to detect such data).

FIG. 13 schematically illustrates an intra-prediction process in which a sample 900 of a block or region 910 of samples is derived from other reference samples 920 of the same image according to a direction 930 defined by the intra-prediction mode associated with that sample. The reference samples 920 in this example come from blocks above and to the left of the block 910 in question and the predicted value of the sample 900 is obtained by tracking along the direction 930 to the reference samples 920. The direction 930 might point to a single individual reference sample but in a more general case an interpolated value between surrounding reference samples is used as the prediction value. Note that the block 910 could be square as shown in FIG. 13 or could be another shape such as rectangular.

FIGS. 14 and 15 schematically illustrate a previously proposed reference sample projection process.

In FIGS. 14 and 15, a block or region 1400 of samples to be predicted is surrounded by linear arrays of reference samples from which the intra prediction of the predicted samples takes place. The reference samples 1410 are shown as shaded blocks in FIGS. 14 and 15, and the samples to be predicted are shown as unshaded blocks. Note that an 8×8 block or region of samples to be predicted is used in this example, but the techniques are applicable to variable block sizes and indeed block shapes.

As mentioned, the reference samples comprise at least two linear arrays in respective orientations with respect to the current image region of samples to be predicted. For example, the linear arrays may be an array or row 1420 of samples above the block of samples to be predicted and an array or column 1430 of samples to the left of the block of samples to be predicted.

As discussed above with reference to FIG. 13, the reference sample arrays can extend beyond the extent of the block to be predicted, in order to provide for prediction modes or directions within the range indicated in FIGS. 10-12. Where necessary, if previously decoded samples are not available for use as reference samples at particular reference sample positions, other reference samples can be re-used at those missing positions. Reference sample filtering processes can be used on the reference samples.

FIG. 14 schematically illustrates the operation of a CABAC entropy encoder.

The CABAC encoder operates in respect of binary data, that is to say, data represented by only the two symbols 0 and 1. The encoder makes use of a so-called context modelling process which selects a “context” or probability model for subsequent data on the basis of previously encoded data. The selection of the context is carried out in a deterministic way so that the same determination, on the basis of previously decoded data, can be performed at the decoder without the need for further data (specifying the context) to be added to the encoded datastream passed to the decoder.

Referring to FIG. 14, input data to be encoded may be passed to a binary converter 1400 if it is not already in a binary form; if the data is already in binary form, the converter 1400 is bypassed (by a schematic switch 1410). In the present embodiments, conversion to a binary form is actually carried out by expressing the quantised DCT coefficient data as a series of binary “maps”, which will be described further below.

The binary data may then be handled by one of two processing paths, a “regular” and a “bypass” path (which are shown schematically as separate paths but which, in embodiments of the invention discussed below, could in fact be implemented by the same processing stages, just using slightly different parameters). The bypass path employs a so-called bypass coder 1420 which does not necessarily make use of context modelling in the same form as the regular path. In some examples of CABAC coding, this bypass path can be selected if there is a need for particularly rapid processing of a batch of data, but in the present embodiments two features of so-called “bypass” data are noted: firstly, the bypass data is handled by the CABAC encoder (950, 1460), just using a fixed context model representing a fixed, for example 50%, probability (otherwise referred to as “equiprobable” or “EP” bins); and secondly, the bypass data relates to certain categories of data, one particular example being coefficient sign data. Otherwise, the regular path is selected by schematic switches 1430, 1440 operating under the control of control circuitry 1435. This involves the data being processed by a context modeler 1450 followed by a coding engine 1460.

The entropy encoder shown in FIG. 14 encodes a block of data (that is, for example, data corresponding to a block of coefficients relating to a block of the residual image) as a single value if the block is formed entirely of zero-valued data. For each block that does not fall into this category, that is to say a block that contains at least some non-zero data, a “significance map” is prepared. The significance map indicates whether, for each position in a block of data to be encoded, the corresponding coefficient in the block is non-zero. The significance map data, being in binary form, is itself CABAC encoded. The use of the significance map assists with compression because no data needs to be encoded for a coefficient with a magnitude that the significance map indicates to be zero. Also, the significance map can include a special code to indicate the final non-zero coefficient in the block, so that all of the final high frequency/trailing zero coefficients can be omitted from the encoding. The significance map is followed, in the encoded bitstream, by data defining the values of the non-zero coefficients specified by the significance map.

Further levels of map data are also prepared and are encoded. An example is a map which defines, as a binary value (1=yes, 0=no) whether the coefficient data at a map position which the significance map has indicated to be “non-zero” actually has the value of “one”. Another map specifies whether the coefficient data at a map position which the significance map has indicated to be “non-zero” actually has the value of “two”. A further map indicates, for those map positions where the significance map has indicated that the coefficient data is “non-zero”, whether the data has a value of “greater than two”. Another map indicates, again for data identified as “non-zero”, the sign of the data value (using a predetermined binary notation such as 1 for +, 0 for −, or of course the other way around).

In embodiments of the invention, the significance maps and the other maps are allocated in a predetermined manner either to the CABAC encoder or to the bypass encoder, and are all representative of different respective attributes or value ranges of the same initial data items. In one example, at least the significance map is CABAC encoded and at least some of the remaining maps (such as the sign data) are bypass encoded. Accordingly, each data item is split into respective subsets of data and the respective subsets are encoded by first (for example, CABAC) and second (for example, bypass) encoding systems. The nature of the data and of the CABAC and bypass encoding is such that for a predetermined quantity of CABAC encoded data, a variable quantity of zero or more bypass data is generated in respect of the same initial data items. So, for example, if the quantised, reordered DCT data contains substantially all zero values, then it may be that no bypass data or a very small quantity of bypass data is generated, because the bypass data concerns only those map positions for which the significance map has indicated that the value is non-zero. In another example, in quantised reordered DCT data having many high value coefficients, a significant quantity of bypass data might be generated.

In embodiments of the invention, the significance map and other maps are generated from the quantised DCT coefficients, for example by the scan unit 360, and is subjected to a zigzag scanning process (or a scanning process selected from zigzag, horizontal raster and vertical raster scanning according to the intra-prediction mode) before being subjected to CABAC encoding.

In general terms, CABAC encoding involves predicting a context, or a probability model, for a next bit to be encoded, based upon other previously encoded data. If the next bit is the same as the bit identified as “most likely” by the probability model, then the encoding of the information that “the next bit agrees with the probability model” can be encoded with great efficiency. It is less efficient to encode that “the next bit does not agree with the probability model”, so the derivation of the context data is important to good operation of the encoder. The term “adaptive” means that the context or probability models are adapted, or varied during encoding, in an attempt to provide a good match to the (as yet uncoded) next data.

Using a simple analogy, in the written English language, the letter “U” is relatively uncommon. But in a letter position immediately after the letter “Q”, it is very common indeed. So, a probability model might set the probability of a “U” as a very low value, but if the current letter is a “Q”, the probability model for a “U” as the next letter could be set to a very high probability value.

CABAC encoding is used, in the present arrangements, for at least the significance map and the maps indicating whether the non-zero values are one or two. Bypass processing—which in these embodiments is identical to CABAC encoding but for the fact that the probability model is fixed at an equal (0.5:0.5) probability distribution of 1s and 0s, is used for at least the sign data and the map indicating whether a value is >2. For those data positions identified as >2, a separate so-called escape data encoding can be used to encode the actual value of the data. This may include a Golomb-Rice encoding technique.

The CABAC context modelling and encoding process is described in more detail in WD4: Working Draft 4 of High-Efficiency Video Coding, JCTVC-F803_d5, Draft ISO/IEC 23008-HEVC; 201x(E) 2011-10-28.

Referring now to FIGS. 15 and 16, an entropy encoder forming part of a video encoding apparatus comprises a first encoding system (for example an arithmetic coding encoding system such as a CABAC encoder 1500) and a second encoding system (such as a bypass encoder 1510), arranged so that a particular data word or value is encoded to the final output data stream by either the CABAC encoder or the bypass encoder but not both. In embodiments of the invention, the data values passed to the CABAC encoder and to the bypass encoder are respective subsets of ordered data values split or derived from the initial input data (the reordered quantised DCT data in this example), representing different ones of the set of “maps” generated from the input data.

The schematic representation in FIG. 15 treats the CABAC encoder and the bypass encoder as separate arrangements. This may well be the case in practice, but in another possibility, shown schematically in FIG. 16, a single CABAC encoder 1620 is used as both the CABAC encoder 1500 and the bypass encoder 1510 of FIG. 15. The encoder 1620 operates under the control of an encoding mode selection signal 1630, so as to operate with an adaptive context model (as described above) when in the mode of the CABAC encoder 1500, and to operate with a fixed 50% probability context model when in the mode of the bypass encoder 1510.

A third possibility combines these two, in that two substantially identical CABAC encoders can be operated in parallel (similar to the parallel arrangement of FIG. 15) with the difference being that the CABAC encoder operating as the bypass encoder 1510 has its context model fixed at a 50% probability context model.

The outputs of the CABAC encoding process and the bypass encoding process can be stored (temporarily at least) in respective buffers 1540, 1550. In the case of FIG. 16, a switch or demultiplexer 1660 acts under the control of the mode signal 1630 to route CABAC encoded data to the buffer 1550 and bypass encoded data to the buffer 1540.

Therefore FIGS. 14-16 provide examples of an entropy encoder configured to selectively encode data items representing image data by an encoding system selected from a CABAC system and a bypass system.

FIGS. 17 and 18 schematically illustrate examples of an entropy decoder forming part of a video decoding apparatus. Referring to FIG. 17, respective buffers 1710, 1700 pass data to a CABAC decoder 1730 and a bypass decoder 1720, arranged so that a particular encoded data word or value is decoded by either the CABAC decoder or the bypass decoder but not both. The decoded data are reordered by logic 1740 into the appropriate order for subsequent decoding stages.

The schematic representation in FIG. 17 treats the CABAC decoder and the bypass decoder as separate arrangements. This may well be the case in practice, but in another possibility, shown schematically in FIG. 18, a single CABAC decoder 1850 is used as both the CABAC decoder 1730 and the bypass decoder 1720 of FIG. 17. The decoder 1850 operates under the control of a decoding mode selection signal 1860, so as to operate with an adaptive context model (as described above) when in the mode of the CABAC decoder 1730, and to operate with a fixed 50% probability context model when in the mode of the bypass encoder 1720.

As before, a third possibility combines these two, in that two substantially identical CABAC decoders can be operated in parallel (similar to the parallel arrangement of FIG. 17) with the difference being that the CABAC decoder operating as the bypass decoder 1720 has its context model fixed at a 50% probability context model.

In the case of FIG. 18, a switch or multiplexer 1870 acts under the control of the mode signal 1860 to route CABAC encoded data to the decoder 1850 from the buffer 1700 or the buffer 1710 as appropriate.

Therefore FIGS. 14-16 provide examples of an entropy decoder configured to selectively decode data items representing image data by a decoding system selected from a CABAC system and a bypass system.

Amongst the data items which can be generated, encoded and decoded in an image data handling system are instances of (i) a first data item indicating whether a multi-direction linear model (MDLM) chroma mode applies to a current image region; (ii) a second data item indicating a processing direction for operation of the MDLM chroma mode (such as using samples above or to the left); and (iii) a flag indicating a planar intra-prediction mode.

In previously proposed arrangements (such as that defined by the latest specification version available at the priority date of the present application, namely JVET-N1001-v8 (draft 5), Bross et al, these items (i)-(iii) are handled as CABAC encoded data. In particular, in respect of items (i) and (ii), the two data items are handled using a common CABAC context (see 9.5.4.2.1 and in particular table 9.17 of the JVET document cited above.

The use of a common CABAC context for data items (i) and (ii) can lead to potential inefficiencies.

For example, the efficient use of a CABAC context is based upon the context tracking the likelihood of particular outcomes of the encoded variable, which is achieved by updating the context in response to encoding the variable. In the situation of the use of MDLM chroma mode, while the actual use of this mode may have a particular likelihood for a given image or image region, once it has been decided at the encoder to use the MDLM chroma mode (data item (i)) the likelihood of choosing a particular direction of operation (data item (ii)) tends to be roughly 50:50. Therefore, by using a common context for data items (i) and (ii), any potential coding efficiency benefit achieved by tracking the likelihood of the use of MDLM chroma mode is potentially undone or at least partially counteracted by also tracking with the same context the substantially equally likely use of the different processing directions (data item (ii)). Either the resulting context variable is less efficient than it could be for encoding the data item (i), or it is inappropriate for encoding the substantially equally likely data item (ii).

In an example arrangement, the first and second data items can be expressed as follows, with reference to so-called intra_chroma_pred_mode values 4-6 as defined by JVET-N1001-v8 (draft 5):

first data item flag indicating LM flag indicating intra_chroma_pred_mode Chroma or not MDLM or not second data item 4 CABAC context 1 Coding method 2 (not present) (LM Chroma mode = yes but MDLM = no) 5 CABAC context 1 Coding method 2 Coding method 3 (LM Chroma mode = yes and MDLM direction = “left”) 6 CABAC context 1 Coding method 2 Coding method 3 (LM Chroma = yes and MDLM direction = “above”)

In some example arrangements, Coding method 2 is bypass coding using EP bins. Therefore, the data item (i) which encompasses an indication that LM Chroma mode is in use (noting that MDLM is not applicable when LM Chroma mode is not in use) and an indication of whether MDLM is also in use, uses at least the CABAC context 1.

In some other example arrangements, Coding method 2 is CABAC coding using a second CABAC context 2. Therefore, the first data item which encompasses an indication that LM Chroma mode is in use (noting that MDLM is not applicable when LM Chroma mode is not in use) and an indication of whether MDLM is also in use, uses at least the CABAC context 1, and also uses CABAC context 2. In terms of the expression of the present technique such that the first data item is encoded using at least a first CABAC context, in these example arrangements the first CABAC context may be taken as either Context 1 or Context 2.

In some example arrangements, Coding method 3 uses a further, independent CABAC context but in other examples Coding method 3 represents bypass coding using EP bins.

It will be appreciated that the term “encoding instances of a data item” can imply starting with an initial representation of the data item and generated an encoded representation. Similarly the term “decoding instances of a data item” can imply starting with an encoded representation of the data item and generated an output representation.

Therefore, at the encoding side, the entropy encoder is configured to encode instances of the first data item and to encode instances of the second data item independently of the encoding of instances of the first data item. At the decoding side, the entropy decoder is configured to decode instances of the first data item and to decode instances of the second data item independently of the decoding of instances of the first data item.

In at least some of the examples given above, the entropy encoder is configured to encode instances of the first data item by the CABAC encoding system using at least a first CABAC context (for example, by including the use of CABAC context 1) and to encode instances of the second data item independently of the first CABAC context (for example, by Coding method 3).

In at least some examples, the first data item comprises data indicative of whether a linear mode chroma prediction mode applies to a current image region. In at least some examples, the entropy encoder is configured to encode using the first CABAC context at least the data indicative of whether a linear mode chroma prediction mode applies to the current image region, even if (as in some examples) the flag indicating whether MDLM is in use is encoded using a further CABAC context or a bypass method.

Corresponding considerations apply at the decoding side.

In some example embodiments, the entropy encoder of FIG. 14 is configured to encode instances of data item (i) by the CABAC encoding system using a first CABAC context and to encode instances of data item (ii) independently of the first CABAC context.

In example embodiments this is achieved in various ways:

In some embodiments of the present disclosure, instances of data items (i) and (ii) are encoded using CABAC but with separate contexts.

In other embodiments of the present disclosure, instances of data item (i) are encoded using CABAC and the first context, but instances of data item (ii) are encoded as bypass data, for example by a binary arithmetic coding system (for example, 1420) using a fixed 50% probability (EP) model

It is also proposed that in example embodiments, instances of data item (iii), the flag indicating a planar intra-prediction mode, are handled as EP (bypass) bins. This proposal is independent of the arrangements discussed above relating to data items (i) and (ii).

FIG. 19 is a schematic flowchart illustrating an image data encoding method, comprising:

selectively encoding (at a step 1900) data items representing image data by an encoding system selected from a context adaptive binary arithmetic coding (CABAC), encoding system and a bypass encoding system;

in which the data items include (i) a first data item indicating whether a multi-direction linear model (MDLM) chroma mode applies to a current image region; and (ii) a second data item indicating a processing direction for operation of the MDLM chroma mode;

in which the encoding step comprises encoding instances of the first data item and encoding instances of the second data item independently of the encoding of instances of the first data item.

FIG. 20 is a schematic flowchart illustrating an image data decoding method, comprising:

selectively decoding (at a step 2000) data items representing image data by a decoding system selected from a context adaptive binary arithmetic coding (CABAC), decoding system and a bypass decoding system, in which the data items include (i) a first data item indicating whether a multi-direction linear model (MDLM) chroma mode applies to a current image region; and (ii) a second data item indicating a processing direction for operation of the MDLM chroma mode;

in which the decoding step comprises decoding instances of the first data and to decode instances of the second data item independently of the decoding of instances of the first data item.

In each case, embodiments of the disclosure are represented by computer software which, when executed by a computer, causes the computer to carry out the respective method and by a machine-readable non-transitory storage medium which stores such computer software. In the case of encoding methods, embodiments of the disclosure are represented by a data signal comprising coded data generated according to the respective method.

In so far as embodiments of the disclosure have been described as being implemented, at least in part, by software-controlled data processing apparatus, it will be appreciated that a non-transitory machine-readable medium carrying such software, such as an optical disk, a magnetic disk, semiconductor memory or the like, is also considered to represent an embodiment of the present disclosure. Similarly, a data signal comprising coded data generated according to the methods discussed above (whether or not embodied on a non-transitory machine-readable medium) is also considered to represent an embodiment of the present disclosure.

It will be apparent that numerous modifications and variations of the present disclosure are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended clauses, the technology may be practised otherwise than as specifically described herein.

Respective aspects and features are defined by the following set of numbered clauses:

1. Image data encoding apparatus, comprising:

an entropy encoder configured to selectively encode data items representing image data by a context adaptive binary arithmetic coding (CABAC), encoding system or by a bypass encoding system;

in which the data items include (i) a first data item indicating whether a multi-direction linear model (MDLM) chroma mode applies to a current image region; and (ii) a second data item indicating a processing direction for operation of the MDLM chroma mode;

in which the entropy encoder is configured to encode instances of the first data item by the CABAC encoding system using a first CABAC context and to encode instances of the second data item independently of the first CABAC context.

2. The image data encoding apparatus of clause 1, in which the entropy encoder is configured to encode instances of the second data item using the bypass encoding system. 3. The image data encoding apparatus of clause 2, in which the bypass encoding system is a binary arithmetic coding system using a fixed 50% probability context model. 4. The image data encoding apparatus of clause 1, in which the entropy encoder is configured to encode instances of the second data item by the CABAC encoding system using a second CABAC context independent of the first CABAC context. 5. Video storage, capture, transmission or reception apparatus comprising apparatus according to any one of the preceding clauses. 6. An image data encoding method, comprising:

selectively encoding data items representing image data by a context adaptive binary arithmetic coding (CABAC), encoding system or by a bypass encoding system;

in which the data items include (i) a first data item indicating whether a multi-direction linear model (MDLM) chroma mode applies to a current image region; and (ii) a second data item indicating a processing direction for operation of the MDLM chroma mode;

in which the encoding step comprises encoding instances of the first data item by the CABAC encoding system using a first CABAC context and to encode instances of the second data item independently of the first CABAC context.

7. The image data encoding method of clause 6, in which the encoding step comprises encoding instances of the second data item using the bypass encoding system. 8. The image data encoding method of clause 7, in which the bypass encoding system is a binary arithmetic coding system using a fixed 50% probability context model. 9. The image data encoding method of clause 6, in which the encoding step comprises encoding instances of the second data item by the CABAC encoding system using a second CABAC context independent of the first CABAC context. 10 Computer software which, when executed by a computer, causes the computer to carry out the method of any one of clauses 6 to 9. 11. A machine-readable non-transitory storage medium which stores the computer software of clause 10 12. A data signal comprising coded data generated according to the method of any one of clauses 6 to 9. 13. Image data decoding apparatus, comprising:

an entropy decoder configured to selectively decode data items representing image data by a context adaptive binary arithmetic coding (CABAC), decoding system or by a bypass decoding system;

in which the data items include (i) a first data item indicating whether a multi-direction linear model (MDLM) chroma mode applies to a current image region; and (ii) a second data item indicating a processing direction for operation of the MDLM chroma mode;

in which the entropy decoder is configured to decode instances of the first data item by the CABAC decoding system using a first CABAC context and to decode instances of the second data item independently of the first CABAC context.

14. The image data decoding apparatus of clause 13, in which the entropy decoder is configured to decode instances of the second data item using the bypass decoding system. 15. The image data decoding apparatus of clause 14, in which the bypass decoding system is a binary arithmetic coding system using a fixed 50% probability context model. 16. The image data decoding apparatus of clause 13, in which the entropy decoder is configured to decode instances of the second data item by the CABAC decoding system using a second CABAC context independent of the first CABAC context. 17. Video storage, capture, transmission or reception apparatus comprising apparatus according to any one of clauses 13 to 16. 18. An image data decoding method, comprising:

selectively decoding data items representing image data by a context adaptive binary arithmetic coding (CABAC), decoding system or by a bypass decoding system;

in which the data items include (i) a first data item indicating whether a multi-direction linear model (MDLM) chroma mode applies to a current image region; and (ii) a second data item indicating a processing direction for operation of the MDLM chroma mode;

in which the decoding step comprises decoding instances of the first data item by the CABAC decoding system using a first CABAC context and to decode instances of the second data item independently of the first CABAC context.

19. The image data decoding method of clause 18, in which the decoding step comprises decoding instances of the second data item using the bypass decoding system. 20. The image data decoding method of clause 19, in which the bypass decoding system is a binary arithmetic coding system using a fixed 50% probability context model. 21. The image data decoding method of clause 18, in which the decoding step comprises decoding instances of the second data item by the CABAC decoding system using a second CABAC context independent of the first CABAC context. 22 Computer software which, when executed by a computer, causes the computer to carry out the method of any one of clauses 18 to 21. 23. A machine-readable non-transitory storage medium which stores the computer software of clause 18.

Further respective aspects and features are defined by the following additional set of numbered clauses.

1. Image data encoding apparatus, comprising:

an entropy encoder configured to selectively encode data items representing image data by an encoding system selected from a context adaptive binary arithmetic coding (CABAC), encoding system and a bypass encoding system;

in which the data items include (i) a first data item indicating whether a multi-direction linear model (MDLM) chroma mode applies to a current image region; and (ii) a second data item indicating a processing direction for operation of the MDLM chroma mode;

in which the entropy encoder is configured to encode instances of the first data item and to encode instances of the second data item independently of the encoding of instances of the first data item.

2. The image data encoding apparatus of clause 1, in which the entropy encoder is configured to encode instances of the first data item by the CABAC encoding system using at least a first CABAC context and to encode instances of the second data item independently of the first CABAC context. 3. The image data encoding apparatus of clause 1, in which the entropy encoder is configured to encode instances of the first data item by the CABAC encoding system using a first CABAC context and to encode instances of the second data item independently of the first CABAC context. 4. The image data encoding apparatus of any one of the preceding clauses, in which the entropy encoder is configured to encode instances of the second data item using the bypass encoding system. 5. The image data encoding apparatus of any one of the preceding clauses, in which the bypass encoding system is a binary arithmetic coding system using a fixed 50% probability context model. 6. The image data encoding apparatus of clause 2, in which the entropy encoder is configured to encode instances of the second data item by the CABAC encoding system using a second CABAC context independent of the first CABAC context. 7. The image data encoding apparatus of any one of the preceding clauses, in which the first data item comprises data indicative of whether a linear mode chroma prediction mode applies to a current image region. 8 The image data encoding apparatus of any one of clauses 2 to 7 as dependent on clause 2, in which:

the first data item comprises data indicative of whether a linear mode chroma prediction mode applies to a current image region; and

the entropy encoder is configured to encode using the first CABAC context at least the data indicative of whether a linear mode chroma prediction mode applies to the current image region.

9. Video storage, capture, transmission or reception apparatus comprising apparatus according to any one of the preceding clauses. 10. An image data encoding method, comprising:

selectively encoding data items representing image data by an encoding system selected from a context adaptive binary arithmetic coding (CABAC), encoding system and a bypass encoding system;

in which the data items include (i) a first data item indicating whether a multi-direction linear model (MDLM) chroma mode applies to a current image region; and (ii) a second data item indicating a processing direction for operation of the MDLM chroma mode;

in which the encoding step comprises encoding instances of the first data item and encoding instances of the second data item independently of the encoding of instances of the first data item.

11. The image data encoding method of clause 10, in which the encoding step comprises encoding instances of the first data item by the CABAC encoding system using at least a first CABAC context and encoding instances of the second data item independently of the first CABAC context. 12. The image data encoding method of clause 10, in which the encoding step comprises encoding instances of the first data item by the CABAC encoding system using a first CABAC context and encoding instances of the second data item independently of the first CABAC context. 13. The image data encoding method of any one of clauses 10 to 12, in which the encoding step comprises encoding instances of the second data item using the bypass encoding system. 14. The image data encoding method of any one of clauses 10 to 13, in which the bypass encoding system is a binary arithmetic coding system using a fixed 50% probability context model. 15. The image data encoding method of clause 11, in which the encoding step comprises encoding instances of the second data item by the CABAC encoding system using a second CABAC context independent of the first CABAC context. 16. The image data encoding method of any one of clauses 10 to 15, in which the first data item comprises data indicative of whether a linear mode chroma prediction mode applies to a current image region. 17 The image data encoding method of any one of clauses 11 to 16 as dependent upon clause 11, in which:

the first data item comprises data indicative of whether a linear mode chroma prediction mode applies to a current image region; and

the encoding step comprises encoding, using the first CABAC context at least the data indicative of whether a linear mode chroma prediction mode applies to the current image region.

18 Computer software which, when executed by a computer, causes the computer to carry out the method of any one of clauses 10 to 17. 19. A machine-readable non-transitory storage medium which stores the computer software of clause 18. 20. A data signal comprising coded data generated according to the method of any one of clauses 10 to 19. 21. Image data decoding apparatus, comprising:

an entropy decoder configured to selectively decode data items representing image data by a decoding system selected from a context adaptive binary arithmetic coding (CABAC), decoding system and a bypass decoding system;

in which the data items include (i) a first data item indicating whether a multi-direction linear model (MDLM) chroma mode applies to a current image region; and (ii) a second data item indicating a processing direction for operation of the MDLM chroma mode;

in which the entropy decoder is configured to decode instances of the first data item and to decode instances of the second data item independently of the decoding of instances of the first data item.

22. The image data decoding apparatus of clause 21, in which the entropy decoder is configured to decode instances of the first data item by the CABAC decoding system using at least a first CABAC context and to decode instances of the second data item independently of the first CABAC context. 23. The image data decoding apparatus of clause 21, in which the entropy decoder is configured to decode instances of the first data item by the CABAC decoding system using a first CABAC context and to decode instances of the second data item independently of the first CABAC context. 24. The image data decoding apparatus of any one of clauses 21 to 23, in which the entropy decoder is configured to decode instances of the second data item using the bypass decoding system. 25. The image data decoding apparatus of any one of clauses 21 to 24, in which the bypass decoding system is a binary arithmetic coding system using a fixed 50% probability context model. 26. The image data decoding apparatus of clause 23, in which the entropy decoder is configured to decode instances of the second data item by the CABAC decoding system using a second CABAC context independent of the first CABAC context. 27. The image data decoding apparatus of any one of clauses 21 to 26, in which the first data item comprises data indicative of whether a linear mode chroma prediction mode applies to a current image region. 28 The image data decoding apparatus of any one of clauses 22 to 27 as dependent upon clause 22, in which:

the first data item comprises data indicative of whether a linear mode chroma prediction mode applies to a current image region; and

the entropy decoder is configured to decode using the first CABAC context at least the data indicative of whether a linear mode chroma prediction mode applies to the current image region.

29. Video storage, capture, transmission or reception apparatus comprising apparatus according to any one of clauses 21 to 28. 30. An image data decoding method, comprising:

selectively decoding data items representing image data by a decoding system selected from a context adaptive binary arithmetic coding (CABAC), decoding system and a bypass decoding system;

in which the data items include (i) a first data item indicating whether a multi-direction linear model (MDLM) chroma mode applies to a current image region; and (ii) a second data item indicating a processing direction for operation of the MDLM chroma mode;

in which the decoding step comprises decoding instances of the first data and to decode instances of the second data item independently of the decoding of instances of the first data item.

31. The image data decoding method of clause 30, in which the decoding step comprises decoding instances of the first data item by the CABAC decoding system using at least a first CABAC context and decoding instances of the second data item independently of the first CABAC context. 32. The image data decoding method of clause 30, in which the decoding step comprises decoding instances of the first data item by the CABAC decoding system using a first CABAC context and decoding instances of the second data item independently of the first CABAC context. 33. The image data decoding method of any one of clauses 30 to 32, in which the decoding step comprises decoding instances of the second data item using the bypass decoding system. 34. The image data decoding method of any one of clauses 30 to 33, in which the bypass decoding system is a binary arithmetic coding system using a fixed 50% probability context model. 35. The image data decoding method of clause 30, in which the decoding step comprises decoding instances of the second data item by the CABAC decoding system using a second CABAC context independent of the first CABAC context. 36. The image data decoding method of any one of clauses 30 to 35, in which the first data item comprises data indicative of whether a linear mode chroma prediction mode applies to a current image region. 37 The image data decoding method of any one of clauses 31 to 36 as dependent upon clause 31, in which:

the first data item comprises data indicative of whether a linear mode chroma prediction mode applies to a current image region; and

the decoding step comprises decoding, using the first CABAC context at least the data indicative of whether a linear mode chroma prediction mode applies to the current image region.

38 Computer software which, when executed by a computer, causes the computer to carry out the method of any one of clauses 30 to 37. 39. A machine-readable non-transitory storage medium which stores the computer software of clause 38. 

1-20. (canceled)
 21. An image data decoding apparatus, comprising: processing circuitry configured to selectively decode data items representing image data by a decoding system selected from a context adaptive binary arithmetic coding (CABAC) decoding system and a bypass decoding system, wherein the data items include (i) a first data item indicating whether a multi-direction linear model (MDLM) chroma mode applies to a current image region; and (ii) a second data item indicating a processing direction for operation of the MDLM chroma mode; and wherein the processing circuitry is further configured to decode instances of the first data item and to decode instances of the second data item independently of the decoding of instances of the first data item.
 22. The image data decoding apparatus of claim 21, wherein the processing circuitry is further configured to decode instances of the first data item by the CABAC decoding system using at least a first CABAC context and to decode instances of the second data item independently of the first CABAC context.
 23. The image data decoding apparatus of claim 21, wherein the processing circuitry is further configured to decode instances of the first data item by the CABAC decoding system using a first CABAC context and to decode instances of the second data item independently of the first CABAC context.
 24. The image data decoding apparatus of claim 23, wherein the processing circuitry is further configured to decode instances of the second data item using the bypass decoding system.
 25. The image data decoding apparatus of claim 24, wherein the bypass decoding system is a binary arithmetic coding system using a fixed 50% probability context model.
 26. The image data decoding apparatus of claim 23, wherein the processing circuitry is further configured to decode instances of the second data item by the CABAC decoding system using a second CABAC context independent of the first CABAC context.
 27. The image data decoding apparatus of claim 21, wherein the first data item comprises data indicative of whether a linear mode chroma prediction mode applies to a current image region.
 28. The image data decoding apparatus of claim 22, wherein: the first data item comprises data indicative of whether a linear mode chroma prediction mode applies to a current image region; and the processing circuitry is further configured to decode using the first CABAC context at least the data indicative of whether a linear mode chroma prediction mode applies to the current image region.
 29. A video storage, capture, transmission, or reception apparatus, comprising the apparatus according to claim
 21. 30. An image data decoding method, comprising: selectively decoding data items representing image data by a decoding system selected from a context adaptive binary arithmetic coding (CABAC), decoding system and a bypass decoding system, wherein the data items include (i) a first data item indicating whether a multi-direction linear model (MDLM) chroma mode applies to a current image region; and (ii) a second data item indicating a processing direction for operation of the MDLM chroma mode; and wherein the decoding step comprises decoding instances of the first data and to decode instances of the second data item independently of the decoding of instances of the first data item. 31-38. (canceled)
 39. A machine-readable non-transitory storage medium including computer program instructions which, when executed by a computer, causes the computer to perform the method of claim
 30. 40. An image data encoding apparatus, comprising: processing circuitry configured to selectively encode data items representing image data by an encoding system selected from a context adaptive binary arithmetic coding (CABAC), encoding system and a bypass encoding system, wherein the data items include (i) a first data item indicating whether a multi-direction linear model (MDLM) chroma mode applies to a current image region; and (ii) a second data item indicating a processing direction for operation of the MDLM chroma mode; and wherein the processing circuitry is further configured to encode instances of the first data item and to encode instances of the second data item independently of the encoding of instances of the first data item.
 41. The image data encoding apparatus of claim 40, wherein the processing circuitry is further configured to encode instances of the first data item by the CABAC encoding system using at least a first CABAC context and to encode instances of the second data item independently of the first CABAC context.
 42. The image data encoding apparatus of claim 40, wherein the processing circuitry is further configured to encode instances of the first data item by the CABAC encoding system using a first CABAC context and to encode instances of the second data item independently of the first CABAC context.
 43. The image data encoding apparatus of claim 40, wherein the processing circuitry is further configured to encode instances of the second data item using the bypass encoding system.
 44. The image data encoding apparatus of claim 43, wherein the bypass encoding system is a binary arithmetic coding system using a fixed 50% probability context model.
 45. The image data encoding apparatus of claim 41, wherein the processing circuitry is further configured to encode instances of the second data item by the CABAC encoding system using a second CABAC context independent of the first CABAC context.
 46. The image data encoding apparatus of claim 40, wherein the first data item comprises data indicative of whether a linear mode chroma prediction mode applies to a current image region.
 47. The image data encoding apparatus of claim 41, wherein: the first data item comprises data indicative of whether a linear mode chroma prediction mode applies to a current image region; and the processing circuitry is further configured to encode using the first CABAC context at least the data indicative of whether a linear mode chroma prediction mode applies to the current image region.
 48. A video storage, capture, transmission or reception apparatus, comprising the apparatus according to claim
 40. 49. An image data encoding method, comprising: selectively encoding data items representing image data by an encoding system selected from a context adaptive binary arithmetic coding (CABAC), encoding system and a bypass encoding system, wherein the data items include (i) a first data item indicating whether a multi-direction linear model (MDLM) chroma mode applies to a current image region; and (ii) a second data item indicating a processing direction for operation of the MDLM chroma mode; and wherein the encoding step comprises encoding instances of the first data item and encoding instances of the second data item independently of the encoding of instances of the first data item.
 50. A non-transitory machine-readable storage medium including computer program instructions, which when executed by a computer causes the computer to perform the method of claim
 49. 